The systems behind the queries — how databases model, store, index, and protect data at scale, the layer beneath the SQL you write. The relational model and the kinds of keys; normalization from first through Boyce-Codd normal form, and when to deliberately denormalize; data modeling and entity-relationship diagrams; indexing done right (B-tree versus hash, composite and covering indexes, and when an index actually hurts); query planning and the classic N+1 problem; transactions and the ACID properties; the isolation levels and exactly which read anomalies (dirty, non-repeatable, and phantom) each one prevents; locking and multi-version concurrency control; the NoSQL families (document, key-value, column-family, and graph) and what each is genuinely for; the CAP theorem and consistency models; replication and sharding; connection pooling; and caching. Grounded in the PostgreSQL documentation and vendor-neutral distributed-systems concepts, with a teaching explanation after every answer. Complements the SQL skill (writing queries) with the data-layer systems behind them.